Materials used for brake disks in the automotive field are nowadays predominantly steel or gray cast iron, and in aircraft applications, of carbon materials reinforced with carbon fibers (C/C). The properties required of the disk materials are high mechanical stability, heat resistance, hardness and wear resistance in the friction pair of the brake. The use temperature of gray cast iron brake disks used hitherto is limited by the melting point of the material. The temperature at which mechanical failure occurs is, depending on the stress, significantly below the melting point. Furthermore, there is a risk of cracking of the disks due to transformation of the metallic microstructure on heating. The use of fiber-reinforced ceramic as a material for brake disk applications has been found to be a solution to these problems. Materials based on silicon carbide reinforced with carbon fibers (C/SiC) in particular have been found useful for this application. The advantages of this material are their lower density (thus reduced weight for a given volume), their high hardness and heat resistance up to about 1400° C. and, not least, the extremely high wear resistance. The significantly reduced density of brake disks made of these C/SiC materials improves comfort and safety by reduction of the unsprung masses in motor vehicles, and also economics in aircraft applications. The high hardness and wear resistance of C/SiC components allows to achieve far longer operating lives compared to previously customary materials based on C/C or metal.
Processes for producing C/SiC components have been known from, for example, DE-A 198 56 721, DE-C 197 11 829 and DE-A 197 10 105 and comprise, inter alia, the following steps:                preparation of a pressable mixture or formable fiber composition comprising, firstly, carbon-containing fibers or fiber bundles which may be coated and, secondly, fillers and/or thermally curable binders such as resins and/or pitch,        shaping of the formable fiber composition or the pressable mixture under pressure and curing at elevated temperature and carbonization of the carbon-containing fillers and binders to produce a shaped body, in particular a shaped body comprising carbon reinforced with carbon fibers (C/C) and, if appropriate, subsequent graphitization,        infiltration of at least an outer layer of the shaped body with a silicon melt and at least partial reaction with the carbon in the shaped body to produce SiC, thus forming a shaped body which comprises, at least in the outer layer, a composite ceramic composed of carbon-containing fibers embedded in a matrix comprising predominantly SiC, Si and C (hereinafter referred to as C/SiC).        
In the following, the term C/SiC also encompasses the material variant in which, as described above, only an outer layer is silicized.
The term “formable fiber composition” encompasses both the fiber-containing press moulding compositions which typically comprise short fibers or short fiber bundles and also fibre mats, woven fabrics or nonwovens which can be processed, for example, by the prepreg technique. The latter can also, in particular, be shaped virtually without or completely with very little or no pressure. Customary production processes also include those in which the C/C body is post-densified via the liquid or gas phase with carbon precursors, namely substances which form carbon on heating in the absence of oxidizing media, or by means of carbon, or the matrix comprising predominantly SiC, Si and C is produced by gas-phase infiltration (CVD, chemical vapor deposition, or CVI, chemical vapor infiltration) or by pyrolysis of Si-containing preceramic polymers.
Present-day metallic brake disks frequently have ventilation slits or ventilation channels through which air flows within the disk so as to reduce the temperature of the disk and decrease wear of the friction lining under high stress. Such ventilation channels are also employed in brake disks based on C/SiC, particularly to lower the temperature so as to spare the brake linings and further components of the system.
One process for producing friction units of C/C—SiC material having ventilation channels, hollow spaces and recesses in which a structured porous carbon body close to the final shape is infiltrated with liquid silicon has been known from EP-B 0 788 468. This process makes use of the fact that liquid silicon infiltration and formation of the Si- and SiC-rich composite matrix occurs virtually without changes to the geometry of the C/C intermediate body, so that the hollow spaces and recesses can be produced in the relatively soft and readily machinable C/C intermediate body and not only in the very hard C/C—SiC composite ceramic.
In DE-C 198 24 571, a further process is proposed for producing hollow spaces in a workpiece comprising C/SiC composite ceramics. The hollow spaces are formed in manufacture of the preform by pressing using cores of silicon, silicon alloys or Si/BN mixtures. The cores are not removed from the preform until the step of infiltration with liquid silicon, and serve as a source of silicon for the silicization. Before silicization, the preform has to be heated and converted into a C/C intermediate body, with decomposition of the organic constituents, for example binders, and shrinking of the preform. This shrinkage leads to the preform shrinking onto the silicon-containing cores which in turn additionally undergo a thermal expansion due to heating. In general, undesirable stresses or even fractures occur in the preform as a result, which have to be avoided.
In the thermal curing of the press moulding or formable fiber compositions, which is generally carried out simultaneously with application of pressure, the heat required is usually introduced into the workpiece from the outside. This is achieved by heating the mold, the press or the punch, or by introducing the workpieces into a furnace. Here, the outer region of the workpiece is heated to higher temperatures than the inner region, so that the heat can be transported into the interior of the workpiece by means of the temperature gradient. This nonuniform heating can lead to stresses in the workpiece; the chemical and physical processes occurring in the outer zone can even lead, for example, to gases liberated during heating and the chemical reactions occurring as a result not being able to escape through the outer zone and contributing to rupture of the workpiece or to cracks in the workpiece.
The German patent application No. DE-A 101 64 231, filed at the same time discloses the use of press moulding compositions which, at least during pressing, have an electrical conductivity of at least 0.1 S/m and are heated by passing electric current through them so as to generate Joule heat before, during or after pressing. However, if attempts are made to apply such a process to green bodies or pressing compositions which contain electrically insulating mold cores, it is observed that local overheating can occur due to the nonuniform cross sections of the conductive regions and ruin the desired effect of uniform heating of the workpiece. Likewise, mold cores having an excessively high conductivity are also unsuitable since they short-circuit the heating current without producing a sufficient heating effect.
It is therefore an object of the invention to provide a process suitable for producing hollow bodies comprising fiber-reinforced composites which are subsequently converted into shaped bodies comprising fiber-reinforced carbide ceramic by infiltration with liquid metals, in particular liquid silicon and subsequent reaction.